Biofuels for transportation are attractive replacements for gasoline and are rapidly penetrating fuel markets as low concentration blends. Biofuels, derived from natural sources, are more environmentally sustainable than those derived from fossil resources (such as gasoline), their use allowing a reduction in the levels of so-called fossil carbon dioxide (CO2) gas that is released into the atmosphere as a result of fuel combustion. In addition, biofuels can be produced locally in many geographical areas, and can act to reduce dependence on imported fossil energy resources.
Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol was expected to reach 27.2 billion gallons by 2012 and the global market for the fuel ethanol industry has also been predicted to grow sharply in future. This growth is mainly due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends, the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, and as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
Butanediols including 1,2-butanediol, 1,3-butanediol, 1,4-butanediol and 2,3-butanediol may be used as an automotive fuel additive. They may also be relatively easily transformed into a number of other potentially higher value and/or higher energy products. For example, 2,3-butanediol may be readily converted in a two step process into an eight-carbon dimer which can be used as aviation fuel.
2,3-Butanediol derives its versatility from its di-functional backbone, i.e., 2 hydroxyl groups are located at vicinal C-atoms allowing the molecule to be transformed quite easily into substances such as butadiene, butadione, acetoin, methylethyl ketone etc. These chemical compounds are used as base molecules to manufacture a vast range of industrially produced chemicals.
In addition, 2,3-butanediol may be used as a fuel in an internal combustion engine. It is in several ways more similar to gasoline than it is to ethanol. As the interest in the production and application of environmentally sustainable fuels has strengthened, interest in biological processes to produce 2,3-butanediol (often referred to as bio-butanol) has increased.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. 2,3-Butanediol can also be produced by microbial fermentation of carbohydrate containing feedstock (Syu M J, Appl Microbiol Biotechnol 55:10-18 (2001), Qin et al., Chinese J Chem Eng 14(1):132-136 (2006)). However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into biofuel products.
Carbon monoxide (CO) is a major free energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually.
It has long been recognised that catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. However, micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as their sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products such as CO2, H2, methane, n-butanol, acetic acid and ethanol.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Abrini et al, Archives of Microbiology 161, pp 345-351 (1994)).
However, biofuel production by micro-organisms by fermentation of gases is always associated with co-production of acetate and/or acetic acid as a by-product. This acetate/acetic acid has the potential to inhibit the reaction and is normally required to be removed from the fermentation broth. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to Green House Gas (GHG) emissions.
Fermentation of gaseous substrates to produce products such as ethanol and 2,3-butanediol is typically carried out in a bioreactor containing a liquid fermentation broth. The broth contains microorganisms and nutrients for their growth. Over time, the nutrients (including the gaseous substrate itself) are converted to desirable products but undesirable by-products and cell debris are also produced that may be toxic to the microorganism. Both desirable and undesirable products may inhibit fermentation efficiency, particularly when present in high concentrations.
In order to recover desirable products and reduce reaction inefficiencies brought about by inhibition of the fermentation reaction, the broth is removed from the bioreactor in a continuous or batch process and replaced with fresh nutrient medium. The desirable products are typically extracted from the broth by way of standard extraction methods such as fractional distillation and extractive fermentation. However, these known methods for extracting organic metabolites from fermentation solutions suffer a number of problems.
Solvent extraction systems often exhibit poor partition ratios when applied to weak organic broths thus making separation difficult. Salt saturation can improve the partition ratios but complicates the extraction process by requiring the removal of the salts from the waste aqueous and dramatically increases consumable costs if the salts cannot be recovered for reuse. Liquid pressure membranes (such as Reverse Osmosis and nanofiltration membranes) do not show sufficiently high rejection for short chained alcohols, diols, and organic acids. Neither hydrophobic nor hydrophilic membranes can be manufactured with tight enough molecular weight cut-offs to exhibit clear separation and both membrane types show severe particulate fouling in fermentation broths, requiring rigorous pre-filtration.
Distillation is currently the primary method for continuous, high purity organic recovery. However, distillation is limited to being used with organic products with lower boiling points than water and without unfavorable azeotropes. Separation of 2,3-butanediol from an aqueous solution by distillation is costly and difficult due to the high boiling point of 2,3-butanediol (180-184° C.) and high affinity of water. Distillation of ethanol from a fermentation broth yields an azeotropic mixture of ethanol and water (i.e. 95% ethanol and 5% water) that cannot be resolved by distillation and requires further steps and technology to separate effectively.
Acetic acid is typically removed by filtration of the broth to remove suspended organic matter followed by passing the broth through an activated charcoal column to adsorb the acetate. This process requires that the pH of the fermentation broth is reduced to less than about 3 before it is passed through the activated charcoal column, to convert the majority of the acetate to the acetic acid form. This method of removal is undesirable as it requires further steps and the addition of pH modifying chemicals to the broth.
Known methods of product recovery are often not appropriate (or are inefficient, in terms of costs and/or energy consumption and/or proprotion of product recovered) to recover major classes of organic products that can be manufactured through fermentation systems, including 2,3-butanediol and acetic acid. Recovery is therefore a bottle-neck for commercially viable production of biofuels using microbial fermentation and there is a need for novel technologies to improve recovery in a more efficient and cost-effective manner.
It is an object of the present invention to provide a process and a fermentation system that overcomes or ameliorates at least one of the disadvantages of the prior art, or at least to provide the public with a useful choice.